Field of the Invention
Embodiments of the invention generally relate to methods and apparatus for depositing nanowires via electrospinning.
Description of the Related Art
In solar, display, and touch screen technologies, transparent conductive oxide (TCO) films are used as electrodes to provide low-resistance electrical contact to a device's active layers while also allowing the passage of light to and from the active layers. However, TCO films possess a number of disadvantages that reduce the absolute efficiency of the device in which the TCO film is utilized. For example, deposition of TCO films requires a balancing of optical transparency and sheet resistance. Thicker films or higher doping levels in the TCO films results in higher conductivities but a reduction in the optical transmission of light. Additionally, the use of TCO films in a device may also require the utilization of additional non-active film layers which can further reduce the absorption of light. Furthermore, TCO films are relatively expensive.
As an alternative to TCO films in devices, the use of metallic nanowires has been proposed. One method of depositing the metallic nanowires is electrospinning. The metallic nanowires are generally deposited onto a substrate surface in a random pattern. Electrospinning includes applying a high voltage to a metallic capillary containing a deposition material including a polymer and a metal. The voltage applied to the capillary creates an electric field sufficient to overcome the surface tension of the deposition material, causing ejection of a thin jet of the deposition material onto a substrate. The deposition material is allowed to deposit on the substrate surface in a random orientation, which is generally dictated by the charged deposition material's affinity for the grounded substrate.
After the material is deposited on the substrate, the deposition material is then annealed to remove volatile polymer components. The remainder of the deposition material is reduced using a reducing agent, such as hydrogen gas, to leave a conductive metal (e.g., a nanowire) on the surface of the substrate. However, due to the random deposition of the nanowires on the substrate, the nanowire pattern does not have a uniform thickness or conductivity, thereby adversely affecting device performance.
Therefore, there is a need for methods and apparatus for aligning nanowires deposited by an electrospinning process.